A study was made of the 131I radioisotope absorption by the thyroid gland, thyroxin binding by a number of tissues, and also protein-bound iodine (PBI) content in the blood plasma of albino rats after the application of alternating magnetic field (AMF)--industrial frequency 200 Ersted voltage--of various duration and periodicity. Plasma PBI increased in response to the 15-minute AMF application. When the exposure was increased to 6.5 hours and especially to 24 hours there was a reduction of the PBI level and of the process of 131I-thyroxin binding by the tissues of the testes, heart, liver, and spleen. The AMF application for 6.5 hours a day for 5 days resulted in a significant elevation of 131I in the thyroid gland and of the PBI, but the 131I-thyroxin binding by the tissues decreased markedly. It is suggested that functional condition of the thyroid gland and the tissue reaction to thyroxin altered depending on the AMF duration and periodicity of action.